Introduced in 1979, Modbus has been the de facto standard ever since. Modbus is an application layer messaging protocol. Placed at level 7 of the OSI model, it provides client/server communication between devices connected via different types of communication buses or communication media. Modbus is the most widely used ICS protocol, mainly because it's a proven and reliable protocol, simple to implement, and open to use without any royalties:
On the left-hand side in the preceding figure, we see Modbus communicating over serial (RS-232 or RS-485). The same application layer protocol is used for communicating over Ethernet, as shown on the right-hand side.
The Modbus protocol is built upon a request and reply model. It uses Function Code in combination with a data section. The Function Code specifies which service is requested for or responded to and the data section provides the data that applies to the function. The Function Code and the data sections are specified...